Conventionally, an uninterruptible power supply device is widely used as a power supply device for stably supplying AC power to a fundamental load such as a computer system. As shown in, for example, Japanese Patent Laying-Open No. 7-298516 (PTL 1), an uninterruptible power supply device generally includes a converter for converting commercial AC power into DC power, and an inverter for converting DC power into AC power for supply to a load. In a normal state where commercial AC power is supplied, DC power generated in the converter is supplied to the inverter and is also stored in a battery. In a power interruption state where supply of commercial AC power is interrupted, the converter is stopped operating, and DC power is supplied from the battery to the inverter.